1. Technical Field
This patent application relates to radio frequency and microwave frequency filters and in particular to a tunable filter using transmission line elements and one or more shunt capacitors. In one implementation, the shunt capacitors may be implemented as a multi-finger mechanically tunable structure.
2. Background Information
Electronic devices such as smart phones, tablets, laptop computers, and the like are now typically expected to communicate using many different communication protocols that operate over a wide range of radio and microwave frequencies. At the same time these devices need to be a small and inexpensive as possible. The ability to accurately filter signals containing unwanted frequencies becomes a critical aspect of designing these products.
Filters may be classed based on how they treat the input signal. In a low pass filter, high frequencies in the input signal are attenuated, and the filter has little attenuation below the so-called cutoff frequency. Other filter classes include band pass, high pass, band stop and pass band. There are also other design considerations for filters, with their frequency responses depending upon design topology, the order of the filter, and other considerations.
A passive filter is a circuit having a topology that includes only passive components such as inductors, capacitors, and resistors. Passive filters are typically built from two port networks or sections. Multiple sections may be connected in a cascade or daisy-chain consisting of either repeats of the same section or completely different sections. The topology may involve L-sections, T sections, Pi-sections, C sections; each section providing a either unbalanced or balance configuration. Ladder networks may also be used.